1. Field of the Invention
The present invention relates to the formation of integrated circuits, more particularly to the deposition of thin films on germanium substrates.
2. Description of the Related Art
Gate dielectrics comprising high-k oxide deposited on silicon have been shown to provide the low leakage current and low equivalent oxide thickness (EOT) values required for the down-scaling of semiconductor feature size. The tradeoff, however, is much decreased mobility. Both the hole and electron mobility in germanium is higher than in silicon (Table 1). As a result, it has been considered as a possible alternative semiconductor for MOSFET's. In addition, it has been suggested that the narrower band gap (0.66 vs 1.12 eV) may be beneficial when scaling down operation voltages. The smaller optical band gap for Ge broadens the absorption wavelength spectrum, allowing optoelectronic integration to enhance CMOS functionality.
TABLE 1Selected electrical properties of silicon and germanium.GeSiBand gap [eV]0.661.12electron mobility39001450[cm2/V · s] (at 300 K)Hole mobility1800505[cm2/V · s] (at 300 K)
There are several significant physical and chemical differences between silicon and germanium that need to be taken into account in order to avoid the potential pit falls which might arise when switching processes to a new material. Germanium is between silicon and tin in the periodic table of elements. Therefore, it shares certain properties with both of these elements. Germanium has two stable oxidation states +2 and +4, of which the latter is more stable. In contrast, the +2 oxidation state is uncommon for silicon. In addition, while silicon is a semimetal, germanium has properties closer to metals.
Both hydrous germanium monoxide (GeO.xH2O) and germanium dioxide (GeO2) are amphoteric. That is, they dissolve both in acids and bases. In addition, germanium oxide dissolves in water. On the other hand, SiO2 is chemically very stable and requires particular chemicals, such as HF, to etch. Further, while germanium oxides sublime at relatively low temperature (GeO sublimes at 500° C. (780 torr)), SiO2 has very low vapor pressure. While SiO has some vapor pressure at high temperature, it is formed only in harsh conditions. Thus, GeO can be reduced to germanium at temperatures as low as 500° C. while SiO2 is very stable and can not be reduced to silicon with hydrogen at relatively low temperatures.
Over the years several different oxides have been deposited on Ge, including GeN, GeOxNy, SiO2, ZrO2, HfO2, and Al2O3. These insulators have been deposited by thermal nitridation and oxidation, MBE, CVD and atomic layer deposition (ALD). However, because germanium does not have a stable native oxide like silicon, it has not been possible to obtain films with good electrical properties. The best results to date have been obtained with native Ge oxide pretreated with ammonia followed by ALD of HfO2. This structure had small flat band voltage (Vfb) shift and small hystheresis.
The instability of germanium oxides explains the poor electrical properties obtained so far. A detailed understanding of the chemistry of germanium is important for adjusting process flows to achieve good ALD growth on germanium.